Synthesis and characterization of new heterocyclic Schiff base palladacycles: Ring activation through N-oxide formation

Reaction of the Schiff base ligand derived from 4-pyridinecarboxaldehyde NC₅H₄C(H)N2′,4′,6′-(CH₃)C₆H₂], (1), with palladium(II) acetate in toluene at 60 °C for 24 h gave Pd{NC₅H₄C(H)N2′,4′,6′-(CH₃)C₆H₂]}₂(OCOCH₃)₂], (2), with two ligands coordinated through the pyridine nitrogen. Treatment of the Schiff base ligand derived from 4-pyridinecarboxaldehyde N-oxide, 4-(O)NC₅H₄C(H)N2′,4′,6′-(CH₃)C₆H₂], (4), with palladium(II) acetate in toluene at 75 °C gave the dinuclear acetato-bridged complex Pd{4-(O)NC₅H₃C(H)N2′,4′,6′-(CH₃)C₆H₂]}(OCOCH₃)]₂, (5) with metallation of an aromatic phenyl carbon. Reaction of complex 5 with sodium chloride or lithium bromide gave the dinuclear halogen-bridged complexes Pd{4-(O)NC₅H₃C(H)N2′,4′,6′-(CH₃)C₆H₂]}(Cl)]₂, (6) and Pd{4-(O)NC₅H₃C(H)N2′,4′,6′-(CH₃)C₆H₂]}(Br)]₂, (7), after the metathesis reaction. Reaction of 6 and 7 with triphenylphosphine gave the mononuclear species Pd{4-(O)NC₅H₃C(H)N2′,4′,6′-(CH₃)C₆H₂]}(Cl)(PPh₃)], (8) and Pd{4-(O)NC₅H₃C(H)N2′,4′,6′-(CH₃)C₆H₂]}-(Br)(PPh₃)], (9), as air stable solids. Treatment of 6 and 7 with Ph₂P(CH₂)₂PPh₂ (dppe) in a complex/diphosphine 1:2 molar ratio gave the mononuclear complexes Pd{4-(O)NC₅H₃C(H)N2′,4′,6′-(CH₃)C₆H₂]}(PPh₂(CH₂)₂PPh₂)]Cl], (10), and Pd{4-(O)NC₅H₃C(H)N2′,4′,6′-(CH₃)C₆H₂]}(PPh₂(CH₂)₂PPh₂)]PF₆], (11), with a chelating diphosphine. The molecular structure of complex 9 was determined by X-ray single crystal diffraction analysis.